The subject of the present invention is a hydraulic percussion apparatus, more particularly a hydraulic hammer of simple design which is intended to be easily fitted to various carrier appliances possessing energy sources of different powers.
Taking into account its multipurpose character, a hammer of this kind must be able to operate over a wide range of inlet flows while retaining an efficiency higher than 0.5, efficiency being the ratio between the power supplied to the tool and the input power. This apparatus must operate independently of the return pressure value, which, depending on the carrier appliances, may vary within large proportions.
Finally, this apparatus must generate a weak recoil force during the striking stroke of the piston in order to limit the vibrations transmitted to the carrier equipment, while supplying a constant energy and a high striking frequency in order to ensure good production.
As an example, a hydraulic hammer capable of being fitted to an appliance of 0.8 to 3 tonnes must be able to have the following characteristics:
inlet flow of hydraulic fluid: from 20 to 45 liters per minute. PA1 return line pressure: PA1 recoil force less than 700 daN. PA1 energy per blow: 180 joules. PA1 striking frequency from 600 to 1500 blows per minute. PA1 diameter of the tool of the order of 45 mm with a minimum diameter of the striking piston of 40 mm. PA1 driving pressure: 56 bars, PA1 the input pressure necessary being 56 bars and the maximum pressure of the return circuit being 30 bars, it is possible to calculate the minimum diameter of the large section of the striking piston, which works out at 59 mm, PA1 the kinetic energy of the striking piston being equal to the hydraulic energy supplied during the accelerated stroke, it is possible to calculate the stroke of the striking piston and the amount of oil needed per blow, which is 70 cm.sup.3, PA1 the predetermined striking frequency makes it possible to calculate the required inlet flow: 105 liters per minute. PA1 the interior of the top chamber is at a regulated pressure of a value intermediate between the inlet or high pressure and the outlet or low pressure, PA1 the annular bottom chamber is alternately connected by a distributor to the high-pressure supply circuit during the upward phase of the piston and to the top chamber during the accelerated downward phase of the piston.
from 0 to 30 bars (1 bar=10.sup.5 Pa).
These different parameters are interconnected.
The choice of the striking frequency in dependence on the inlet flow determines in fact the amount of oil under pressure available for an operating cycle, the permissible recoil force determines the maximum pressure that can be applied to the driving section of the striking piston, and the energy per stroke, which is equal to the kinetic energy stored by the piston at the moment of the impact, determines the striking stroke of the piston.
The stress in the steel of the striking piston and of the tool is in fact proportional to the speed of impact of the piston. Since the permissible fatigue stress is known, and since the force applied to the driving section and the kinetic energy are defined, it is easy to calculate the striking stroke required, it being specified that this stroke must be sufficient to achieve the different switchings needed for the distribution to function.
Finally, the amount of oil under pressure available for a cycle must serve not only for the accelerated stroke of the striking piston, but also for the return stroke of said piston, since the distribution of this amount of oil substantially influences the total efficiency of the apparatus.